1. Field of the Invention
This invention pertains to a method and apparatus for optically accessing a preselected character of a multiplicity of discrete characters contained in an array and more particularly for rapidly accessing the preselected one and providing an image thereof at a location independent of the character accessed.
2. Description of the Prior Art
Standard impact printer systems employ a rotating or translating member that contains one or more sets of the character vocabulary. To access the proper character an instant of time is selected, from the total available cycle time, at which the impact of a hammer transfers the desired character from the rotating or translating member to the print out paper. This approach has been employed to produce an optical printer wherein a light beam is directed to a mask, containing transparencies or templates of the character vocabulary, at at the proper time to illuminate the desired character contained therein. The spatially modulated beam resulting therefrom is focused upon a photoconductive drum member of an electrophotographic copier system wherein the character is rendered visible and transferred to a hard copy paper image. This time-domain accessing of characters requires the light beam, which illuminates the rotating character mask, to be pulsed not only at the proper time to select the desired character but in such a fashion as to hold character blurring within acceptable tolerances. Consequently, the average light pulse duty cycle must include a pulse that is sufficiently brief to provide an instantaneous snapshot of the selected character at an interpulse period, of average duration, that is equivalent to one cycle of the entire vocabulary of characters. To prevent blurring the character motion must be less than five percent. Thus, if the printable vocabulary contains 100 symbols an average light pulse duty cycle of 5 .times. 10.sup.-4 is required. This duty cycle dictates at peak power level for the pulse system's light source that is 2,000 times greater than that of a light source in an illumination system which could function with a 100 percent duty cycle.
Time domain accessing for optical printing systems as described above exhibit printing speed limitations. A system employing a character reel that rotates at 3600 rpm and carries two vocabulary sets on its circumference has a printing rate of 120 characters per second or approximately 60 lines of printing per minute. To surmount this printing speed limitation a multiplicity of light sources had been employed. Printing speeds of several thousand lines per minute have been achieved by employing one light source for each printing line on a page. This is a brute force approach that is expensive and which results in a short MTBF. Printing speeds of optical printing systems may be increased by replacing the time dimension accessing procedure with a system that randomly positions the beam to access the character mask. Position accessing refers to the procedure whereby stationary array of characters is addressed by altering the direction of the light beam to strike the desired character template. However, in position accessing, the change in light beam direction produced to address each character template, must be subsequently eliminated so that the final beam position for each character accessed remains unchanged.